drm/amd/display: fix re-enabling stutter for raven

We were overwriting the whole register which was re-enabling
stutter for raven. Now we are reading the register then setting
the values only for pstate.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>